A gunman in the southern state of Alabama went on a shooting rampage, killing at least nine people before turning the gun fatally on himself, Alabama department of public safety said.

Authorities were "responding to a series of at least four shooting incidents involving what is believed to be a single gunman who left at least nine victims dead before he died from a self-inflicted gunshot," it said in a statement.

"This is a very complicated investigation that is unfolding," it added.

The Alabama office of the Federal Bureau of Investigation is also cooperating in the investigation, a spokeswoman told AFP.Police began to uncover the grisly aftermath of the gunman's rage late Tuesday afternoon,when four adults and a child were found shot to death inside a residence in the town of Samson, authorities said.

Then, two more adults were found dead in two separate residences nearby, the statement said.

"It is believed the gunman left that location, traveling on Alabama route 52 where he shot at a state trooper's vehicle, striking the vehicle seven times and slightly wounding the trooper due to broken glass," it said.

The gunman, who was not identified, gunned down another individual at Samson Pipe and Supply and then another victim at a service station before heading to Reliable Metal Products where he "fired an estimated 30-round burst."

The local police chief was grazed by a bullet but escaped serious injury because he was wearing a bullet-resistant vest.

The gunman entered the facility and "within minutes, shots were heard from within Reliable Metal, and law enforcement officers found him dead from what are believed to be self-inflicted gunshots," the statement said.